content: // Protocol Buffers - Google's data interchange format
  // Copyright 2008 Google Inc.  All rights reserved.
  // https://developers.google.com/protocol-buffers/
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  package com.google.protobuf;
  
  /**
   * Abstract base interface for protocol-buffer-based RPC services.  Services
   * themselves are abstract classes (implemented either by servers or as
   * stubs), but they subclass this base interface.  The methods of this
   * interface can be used to call the methods of the service without knowing
   * its exact type at compile time (analogous to the Message interface).
   *
   * <p>Starting with version 2.3.0, RPC implementations should not try to build
   * on this, but should instead provide code generator plugins which generate
   * code specific to the particular RPC implementation.  This way the generated
   * code can be more appropriate for the implementation in use and can avoid
   * unnecessary layers of indirection.
   *
   * @author kenton@google.com Kenton Varda
   */
  public interface Service {
    /**
     * Get the {@code ServiceDescriptor} describing this service and its methods.
     */
    Descriptors.ServiceDescriptor getDescriptorForType();
  
    /**
     * <p>Call a method of the service specified by MethodDescriptor.  This is
     * normally implemented as a simple {@code switch()} that calls the standard
     * definitions of the service's methods.
     *
     * <p>Preconditions:
     * <ul>
     *   <li>{@code method.getService() == getDescriptorForType()}
     *   <li>{@code request} is of the exact same class as the object returned by
     *       {@code getRequestPrototype(method)}.
     *   <li>{@code controller} is of the correct type for the RPC implementation
     *       being used by this Service.  For stubs, the "correct type" depends
     *       on the RpcChannel which the stub is using.  Server-side Service
     *       implementations are expected to accept whatever type of
     *       {@code RpcController} the server-side RPC implementation uses.
     * </ul>
     *
     * <p>Postconditions:
     * <ul>
     *   <li>{@code done} will be called when the method is complete.  This may be
     *       before {@code callMethod()} returns or it may be at some point in
     *       the future.
     *   <li>The parameter to {@code done} is the response.  It must be of the
     *       exact same type as would be returned by
     *       {@code getResponsePrototype(method)}.
     *   <li>If the RPC failed, the parameter to {@code done} will be
     *       {@code null}.  Further details about the failure can be found by
     *       querying {@code controller}.
     * </ul>
     */
    void callMethod(Descriptors.MethodDescriptor method,
                    RpcController controller,
                    Message request,
                    RpcCallback<Message> done);
  
    /**
     * <p>{@code callMethod()} requires that the request passed in is of a
     * particular subclass of {@code Message}.  {@code getRequestPrototype()}
     * gets the default instances of this type for a given method.  You can then
     * call {@code Message.newBuilderForType()} on this instance to
     * construct a builder to build an object which you can then pass to
     * {@code callMethod()}.
     *
     * <p>Example:
     * <pre>
     *   MethodDescriptor method =
     *     service.getDescriptorForType().findMethodByName("Foo");
     *   Message request =
     *     stub.getRequestPrototype(method).newBuilderForType()
     *         .mergeFrom(input).build();
     *   service.callMethod(method, request, callback);
     * </pre>
     */
    Message getRequestPrototype(Descriptors.MethodDescriptor method);
  
    /**
     * Like {@code getRequestPrototype()}, but gets a prototype of the response
     * message.  {@code getResponsePrototype()} is generally not needed because
     * the {@code Service} implementation constructs the response message itself,
     * but it may be useful in some cases to know ahead of time what type of
     * object will be returned.
     */
    Message getResponsePrototype(Descriptors.MethodDescriptor method);
  }
  